The present invention relates to the field of mobile wireless communication systems and more specifically to methods and apparatus for communication with mobile telephone users (cellular and personal communication systems), mobile wireless data communications, two-way paging and other mobile wireless systems.
In a mobile wireless network mobile stations (MS) are typically in communications with one base transceiver station (BTS) through up and down radio links. Such ground-based radio links suffer from strong local variations in path loss mainly due to obstructions and line-of-sight attenuation. As MS move from one point to another, their signal path losses go through shadow fading fluctuations that are determined, among other things, by the physical dimension of the obstructions, antenna heights and MS velocity. These variations in path loss, must be taken into account in the design of the up-link and down-link radio link resource allocation.
While communicating with a specific host BTS, MS are frequently within the communications range of other BTS. Statistically, due to the distribution of physical obstructions, the shadow fading path loss fluctuations to such other BTS tend to be only weakly correlated with the path loss fluctuations on the link between the MS to host BTS link. It is therefore possible that a MS, at anyone time and location, has a lower path loss to a different BTS than the one it is communicating with.
In a conventional wireless network using the GSM standard, the base station controller (BSC) manages the radio link resources of the BTS. These resources are determined by the number of transceivers installed at the BTS and the number of radio channels anyone transceiver can handle. For example, in TDMA standards, a radio channel consists of a frequency and a time slot. In CDMA standards, a radio channel is represented by a frequency and one of a number of orthogonal spreading codes.
A BTS has two principal functions, that of controlling the radio links with all MSs within its cell, and relaying traffic between the BSC and the MSs. Relaying traffic includes receiving down-link traffic from the BSC and broadcasting it to MSs using broadcasters and that of receiving up-link traffic from the MSs using radio receivers called collectors and relaying it to the BSC.
In a mobile wireless network with a BSC, the BSC controls the assignment of the radio link resources (including Broadcasters and Collectors) in the BTSs as well as the operation of the network, and, through the MSC, provides an interface with the Public Switched Telephone Network (PSTN). For generality, the BTS broadcasting and collecting functions can be considered as separate entities. In most existing networks, however, broadcasters (B) and collectors (C) are co-located.
In one example, three base transceiver stations (BTS) include three broadcasters and three collectors where broadcasters and collectors are typically but not necessarily co-located. The broadcasters and collectors have down-links and up-links to the BSC. These links are typically cabled links such as T1/E1 lines. The connection of these links between the broadcasters or collectors with the BSC maybe arranged in various configurations such as a star pattern, a daisy-chain pattern or in any combination of these or other patterns.
When a connection is setup between a MS and the mobile network, a BSC selects the BTS that has the best radio access to the MS. This setup process includes a series of signal transmissions back and forth between the BSC, the BTSs, and the MSs using up-link and down-link radio control channels. The setup process results in the assignment of dedicated radio traffic and control channels for the up-links and down-links for communications between the MSs and the BTSs. Once these connections are set-up, user traffic, also called payload, can be transmitted between the MSs and the BSC. While the connection lasts, the BTS/BSC controls the operation of the radio traffic channels, including power control, frequency hopping, and timing advance. Also, the BTS/BSC continues to use the radio broadcast channels for operation, maintenance and signaling with all other MSs in its cell.
Users (MSs) communicate with collectors via control up-links and traffic up-links and with broadcasters via control down-links and traffic down-links. A particular broadcaster and collector is called the host broadcaster and the host collector for a particular MS. Together, they perform the function of the host BTS for the particular MS.
As MSs move within a cell and as the average path loss between an MS and its serving broadcaster and collector degrades, existing networks reassign the MS to another BTS (with a broadcaster and collector) that has a lower path loss. This process is called handover or handoff. Prior systems distinguish between hard and soft handover. During hard handover, both the control and traffic radio links between the MS and BTS are terminated and new radio links are set-up between the MS and the new BTS using the radio resources assigned to the new BTS. In case of a handoff failure, the MS and BTS reestablish the control and traffic radio link as it existed before the handoff was attempted. This hard handover is used in GSM networks. In CDMA networks, hard and soft handoff is practiced. In soft handoff, the new radio links are setup before the old links are terminated (make before break operation). CDMA allows simultaneous communications of a MS with a number of BTS during soft handoff.
One technique for maintaining low transmit power during the operation of a mobile radio link is dynamic power control. It maybe applied on both the up-link and down-link directions or only in one direction, and it may be performed in an open-loop or closed-loop mode. In open-loop power control mode, the transmit power is determined by system level parameters. In closed-loop power control mode, the power is dynamically set in response to radio link measurements such as distance measurements between the MS and the BTS (as determined by time of arrival measurements), receive signal strength measurements, or error rate measurements.
Another known method to improve network performance is the use of macrodiversity signal combining (also called aggregation). This method uses multiple spaced-apart transmitter/broadcasters and collector/receivers in the BTSs to simultaneously communicate with a MS. The soft handoff practiced in CDMA is such an example. On the down-link, the signal is transmitted from multiple spaced-apart broadcasters using down-link traffic channels. These multiple signals are received by the MS (for example using a rake receiver in CDMA), and combined, to provide a processed signal with a higher level of confidence. On the up-link, multiple spaced-apart receivers/collectors receive the signal transmitted by the MS on up-link traffic channels. These multiple receive signals are then transported to a central location and processed to provide a processed signal with a higher confidence level then any of the individual signals would provide. One disadvantage of macrodiversity combining, when used on the up-link, is the added backhaul associated with transporting the receive signals from multiple collectors to one central location.
In GSM systems, Channel Coding occurs for the Channels using a number of techniques including Block Coding, Data Reordering, Convolutional Coding, Repacking and Interleaving. Interleaving is employed, for example, in the Traffic Channel for Full-rate Speech (TCH/FS), the Fast Associated Control Channel for Full-rate Speech traffic (FACCH/FS) and the Slow Associated Control Channel for Full-rate Speech traffic (SACCH/FS).
For TCH/FS and FACCH/FS, the processing prior to interleaving produces an output block of 456 channel coded bits. To guard against burst errors during transmission, half of the bits within this block of coded bits are interleaved with half of the bits from the previous block of coded bits. The remaining bits are then interleaved with half of the bits from the next block of coded bits. The interleaving process results in the 456 coded bits being spread out over 8 bursts of 114 bits in sub-bursts of 57 bits each. The interleaving algorithm is given by the following two equations, Eqs (1), which define where each of the 456 channel coded bits is placed within the 8 bursts of 114 interleaved bits.Burst #=k mod 8 range {0, 1, 2, . . . 7}Bit #=2[(49 k) mod 57]+[(k mod 8) div 4] range {0, 1, 2, . . . 114}  Eqs (1)where:k=bit number of the 456 channel coded bits range {0, 1, . . . , 455}
The major result of the two interleaving equations is that each of the eight blocks will contain either 57 even channel coded bits or 57 odd channel coded bits from a particular speech block of 456 bits.
Interleaving for SACCH/FS is used to help alleviate the effects of error bursts during transmission. The 456 channel coded bits (228 even bits and 228 odd bits) are interleaved with an algorithm which is similar to the algorithm applied to the traffic channel switch one significant difference. The traffic channel data is interleaved with adjacent frames of data where a 456 bit traffic frame was interleaved with both the preceding traffic frame and the following traffic frame. For the SACCH/FS, the 456 channel coded bits are interleaved among themselves. The interleaving process results in the 456 coded bits being spread out over 4 bursts of 114 bits.
The interleaving algorithm is given by the following two equations, Eqs. (2), which define where each of the 456 channel coded bits is placed within the 4 blocks of 114 interleaved bits.Burst #=k mod 4 range {0, 1, 2, 3}Bit #=2[(49 k) mod 57]+[(k mod 8) div 4] range {0, 1, 2, . . . 114}  Eqs (2)wherek=bit number of the 456 channel coded bits range {0, 1, 2, . . . , 455}
The major result of the two interleaving equations, Eqs (2), is each of the four bursts will contain either 114 even channel coded bits or 114 odd channel coded bits.
Although interleaving is useful for providing some immunity to interfering signal bursts or other channel conditions over interleave periods that are longer than such bursts, such interleaving hampers other processing that has fast operations for improving performance. Fast processing that operates to make changes within times that are shorter than the interleave operation period are hampered because the data at such times in an interleaved order different from the normal order.
In wireless networks, dedicated radio links serve individual MSs and are at times operated at lower power levels. For instance, MSs close to a BTS do not require large transmit power levels and are operated at the minimum level meeting the link quality requirements. The reason for reducing power is to conserve radio band resources to enable reuse of radio resources in as many cells in the network as possible. MSs sharing up-link radio resources generate co-channel interference at their respective BTSs BTSs sharing down-link radio resources generate co-channel interference at MSs.
Shadow fading imposes large fluctuations on the path loss between a particular MS moving in a cell and its serving BTS. At times when the path loss to a BTS is high, a high transmit power is used to maintain the quality of service. At such times, it is likely that the path loss between the particular MS and another BTS is lower because shadow fading effects between a MS and different BTSs are not highly correlated. Therefore, such other BTS can communicate traffic and/or control signals with the particular MS using lower up-link and down-link power levels. By switching the traffic and/or control channel over to such other BTS, the contribution of the particular radio link to the interference level in the network for other MS-BTS links that use the same radio resources is reduced. When such switching is implemented for many radio links in a network, a larger number of links can be operated in the network increasing network capacity without adding radio bandwidth.
The above-identified, cross-referenced application entitled SYSTEM FOR FAST MACRODIVERSITY SWITCHING IN MOBILE WIRELESS NETWORKS takes advantage of the de-correlation of shadow fading effects using fast macrodiversity switching (FMS) to select a BTS with the lowest instantaneous path loss for communicating up-link and down-link channels to a particular MS. In operation, host and assistant BTSs are employed. The host BTS remains in control of the particular MS via its broadcast channel until a handover is carried out. The dedicated channels with the particular MS are routed originally through the host BTS. When another BTS with a lower path loss becomes available, traffic and control channels are routed through such other BTS, which is designated as the assistant BTS for particular channels. As an MS moves through the cell, and as its path and shadow-fading losses change, the dedicated channels are switched among a number of BTSs in the network, including the host BTS. This fast macrodiversity switching continues unless the path loss between the particular MS and the host BTS becomes too high and a handover of the broadcast and dedicated channels is executed.
In the fast macrodiversity switching (FMS) process described, the radio resource used for a broadcast channel (frequency, time slot, code) for the host BTS is not changed while the dedicated channels are switched. The FMS process therefore differs from the handover process. Specifically, in the handover process, both the broadcast and dedicated channels are switched from radio resources assigned to the old BTS to radio resources assigned to the new BTS in accordance with a frequency reuse plan. By way of contrast in the FMS process, the broadcast channel is not switched while the dedicated channels are switched. The time scale of the FMS switching process is fast relative to switching for a handover. Fast macrodiversity switching operates, for example, at switching speeds less than one second and in the range of 0.02 seconds to 0.25 seconds in a GSM embodiment. The FMS process can be done without modification to standard MS operation and also without signaling to a MS.
In an FMS environment where interleaving is present, the combination of interleaving and fast macrodiversity switching causes portions of interleaved data to be split and directed to different locations, that is, to different host or assistant BTSs. When interleaved data is split so as to reside at different locations, the interleaved process is disturbed and will not operate, if at all, in the normal manner.
Accordingly, there is a need for improved processing that permits fast macrodiversity switching in an environment of interleaving that helps achieve the objectives of improved performance and higher density of MSs.